There is a very extensive use of heat-resistant cast alloys in various industries. These alloys have been standardized by the Alloy Castings Institute (ACI) Division of the Steel Founders Society of America and given various designations, such as HH or HP.
The ACI designations use a prefix of H for alloys intended for heat-resistance services. The second letter of the ACI designation indicates the specific grade or alloy type, with a rough alphabetical sequence as nickel content rises. The various grades that have now become standardized are listed in Table I along with their standard compositional limits.
TABLE I __________________________________________________________________________ Standard Cast Heat-Resistant Alloys For Industrial Applications Weight Percent of Elements Cast Cast Heat Resistant Alloys for Industrial Applications Alloy Composition-percent (balance essentially Fe) Desig- Mn Si P S nation Ni Cr C Max. Max. Max. Max. Other Elements __________________________________________________________________________ HF 9-12 19-23 0.20-0.40 2.00 2.00 0.04 0.04 Mo 0.5 max HH 11-14 24-28 0.20-0.50 2.00 2.00 0.04 0.04 Mo 0.5 max. N 0.2 max HI 14-18 26-30 0.20-0.50 2.00 2.00 0.04 0.04 Mo 0.5 max HK 18-22 24-28 0.20-0.60 2.00 2.00 0.04 0.04 Mo 0.5 max HL 18-22 28-32 0.20-0.60 2.00 2.00 0.04 0.04 Mo 0.5 max HN 23-27 19-23 0.20-0.50 2.00 2.00 0.04 0.04 Mo 0.5 max HP 33-37 24-28 0.35-0.75 2.00 2.00 0.04 0.04 Mo 0.5 max HT 33-37 15-19 0.35-0.75 2.00 2.50 0.04 0.04 Mo 0.5 max HU 37-41 17-21 0.35-0.75 2.00 2.50 0.04 0.04 Mo 0.5 max HW 58-62 10-14 0.35-0.75 2.00 2.50 0.04 0.04 Mo 0.5 max __________________________________________________________________________
Numerous non-standard cast heat-resistant alloys have been developed which have increased hot strength and service life at elevated temperatures beyond the levels normally provided by the standard ACI alloys. These alloys provide the option of improved performance over those provided by the ACI standard grades but at greatly increased costs and restricted availability. Such non-standard alloys have contained one or more additional elements as follows: 3 to 15% cobalt, 1.7 to 17% tungsten, 1.2 to 1.65% columbium, 3% molybdenum, and less than 1% zirconium. The iron content of these alloys ranges from about 36% to less than 1%.
Furthermore, in recent decades, the standard grades of wrought steels have been improved by relatively small additions of extra elements with often dramatic improvements in properties. These additions have often been of small enough proportions that they have been called microalloy additions.
High temperature strength, measured as creep rupture strength, is usually the predominant property of interest with respect to ACI standard base alloys. U.S. Pat. No. 4,077,801 of Heyer and Huth discloses microalloy additions to the standard ACI heat-resistant casting alloys of 0.05 to 2% tungsten and 0.05 to less than 1% titanium. These additions are said to produce values of creep rupture strengths in the HH through HP grades at least 5% higher than those of the standard grades.
However, many industrial castings have configurations or designs which tend to cause restriction or resistance to the normal shrinkage that takes place during the final freeze and cool down from their pouring temperatures. The resulting thermal stresses cause cracks in the final castings, called hot cracks or hot tears, while metallic alloys especially prone to this defect are said to be hot short or have hot shortness.
Because of their increased rupture strengths, the alloys described in U.S. Pat. No. 4,077,801 are particularly beneficial in the production of centrifugally cast pipes for heat service. Those alloys have also been successfully used in static casting techniques for castings which are of configurations not very susceptible to hot tearing during freezing from molten metal to the solid finished form. On the other hand, those alloys have not been used successfully for static castings having configurations which restrict normal shrinkage during cool down because they are susceptible to the hot shortness problem.
The retention of the high temperature strength afforded by the microalloyed ACI-type alloys nevertheless remains of special interest when the alloy is employed in such static castings, for example, large castings formed around large cored center cavities, such as heat treating retorts. Therefore, there has remained a need for modifying standard ACI heat-resistant alloys in order to increase their strengths while also avoiding an increase in their tendencies toward hot shortness.